/***********************************************************
Copyright 1991 by Stichting Mathematisch Centrum, Amsterdam, The
Netherlands.

                        All Rights Reserved

Permission to use, copy, modify, and distribute this software and its
documentation for any purpose and without fee is hereby granted,
provided that the above copyright notice appear in all copies and that
both that copyright notice and this permission notice appear in
supporting documentation, and that the names of Stichting Mathematisch
Centrum or CWI not be used in advertising or publicity pertaining to
distribution of the software without specific, written prior permission.

STICHTING MATHEMATISCH CENTRUM DISCLAIMS ALL WARRANTIES WITH REGARD TO
THIS SOFTWARE, INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND
FITNESS, IN NO EVENT SHALL STICHTING MATHEMATISCH CENTRUM BE LIABLE
FOR ANY SPECIAL, INDIRECT OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT
OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.

******************************************************************/

/*
 * regcomp and regexec -- regsub and regerror are elsewhere
 *
 *     Copyright (c) 1986 by University of Toronto.
 *     Written by Henry Spencer.  Not derived from licensed software.
#ifdef MULTILINE
 *     Changed by Guido van Rossum, CWI, Amsterdam
 *     for multi-line support.
#endif
 *
 *     Permission is granted to anyone to use this software for any
 *     purpose on any computer system, and to redistribute it freely,
 *     subject to the following restrictions:
 *
 *     1. The author is not responsible for the consequences of use of
 *             this software, no matter how awful, even if they arise
 *             from defects in it.
 *
 *     2. The origin of this software must not be misrepresented, either
 *             by explicit claim or by omission.
 *
 *     3. Altered versions must be plainly marked as such, and must not
 *             be misrepresented as being the original software.
 *
 * Beware that some of this code is subtly aware of the way operator
 * precedence is structured in regular expressions.  Serious changes in
 * regular-expression syntax might require a total rethink.
 */
#include <stdio.h>
#include "PROTO.h"
#include "malloc.h"
#undef ANY /* Conflicting identifier defined in malloc.h */
#include <string.h>              /* XXX Remove if not found */
#include "regexp.h"
#include "regmagic.h"

#ifdef MULTILINE
/*
 * Defining MULTILINE turns on the following changes in the semantics:
 * 1.  The '.' operator matches all characters except a newline.
 * 2.  The '^' operator matches at the beginning of the string or after
 *     a newline.  (Anchored matches are retried after each newline.)
 * 3.  The '$' operator matches at the end of the string or before
 *     a newline.
 * 4.  A '\' followed by an 'n' matches a newline.  (This is an
 *     unfortunate exception to the rule that '\' followed by a
 *     character matches that character...)
 *
 * Also, there is a new function reglexec(prog, string, offset)
 * which searches for a match starting at 'string+offset';
 * it differs from regexec(prog, string+offset) in assuming
 * that the line begins at 'string'.
 */
#endif

/*
 * The "internal use only" fields in regexp.h are present to pass info from
 * compile to execute that permits the execute phase to run lots faster on
 * simple cases.  They are:
 *
 * regstart    char that must begin a match; '\0' if none obvious
 * reganch     is the match anchored (at beginning-of-line only)?
 * regmust     string (pointer into program) that match must include, or NULL
 * regmlen     length of regmust string
 *
 * Regstart and reganch permit very fast decisions on suitable starting points
 * for a match, cutting down the work a lot.  Regmust permits fast rejection
 * of lines that cannot possibly match.  The regmust tests are costly enough
 * that regcomp() supplies a regmust only if the r.e. contains something
 * potentially expensive (at present, the only such thing detected is * or +
 * at the start of the r.e., which can involve a lot of backup).  Regmlen is
 * supplied because the test in regexec() needs it and regcomp() is computing
 * it anyway.
 */

/*
 * Structure for regexp "program".  This is essentially a linear encoding
 * of a nondeterministic finite-state machine (aka syntax charts or
 * "railroad normal form" in parsing technology).  Each node is an opcode
 * plus a "next" pointer, possibly plus an operand.  "Next" pointers of
 * all nodes except BRANCH implement concatenation; a "next" pointer with
 * a BRANCH on both ends of it is connecting two alternatives.  (Here we
 * have one of the subtle syntax dependencies:  an individual BRANCH (as
 * opposed to a collection of them) is never concatenated with anything
 * because of operator precedence.)  The operand of some types of node is
 * a literal string; for others, it is a node leading into a sub-FSM.  In
 * particular, the operand of a BRANCH node is the first node of the branch.
 * (NB this is *not* a tree structure:  the tail of the branch connects
 * to the thing following the set of BRANCHes.)  The opcodes are:
 */

/* definition  number  opnd?   meaning */
#define        END     0       /* no   End of program. */
#define        BOL     1       /* no   Match "" at beginning of line. */
#define        EOL     2       /* no   Match "" at end of line. */
#define        ANY     3       /* no   Match any one character. */
#define        ANYOF   4       /* str  Match any character in this string. */
#define        ANYBUT  5       /* str  Match any character not in this string. */
#define        BRANCH  6       /* node Match this alternative, or the next... */
#define        BACK    7       /* no   Match "", "next" ptr points backward. */
#define        EXACTLY 8       /* str  Match this string. */
#define        NOTHING 9       /* no   Match empty string. */
#define        STAR    10      /* node Match this (simple) thing 0 or more times. */
#define        PLUS    11      /* node Match this (simple) thing 1 or more times. */
#define        OPEN    20      /* no   Mark this point in input as start of #n. */
                       /*      OPEN+1 is number 1, etc. */
#define        CLOSE   30      /* no   Analogous to OPEN. */

/*
 * Opcode notes:
 *
 * BRANCH      The set of branches constituting a single choice are hooked
 *             together with their "next" pointers, since precedence prevents
 *             anything being concatenated to any individual branch.  The
 *             "next" pointer of the last BRANCH in a choice points to the
 *             thing following the whole choice.  This is also where the
 *             final "next" pointer of each individual branch points; each
 *             branch starts with the operand node of a BRANCH node.
 *
 * BACK                Normal "next" pointers all implicitly point forward; BACK
 *             exists to make loop structures possible.
 *
 * STAR,PLUS   '?', and complex '*' and '+', are implemented as circular
 *             BRANCH structures using BACK.  Simple cases (one character
 *             per match) are implemented with STAR and PLUS for speed
 *             and to minimize recursive plunges.
 *
 * OPEN,CLOSE  ...are numbered at compile time.
 */

/*
 * A node is one char of opcode followed by two chars of "next" pointer.
 * "Next" pointers are stored as two 8-bit pieces, high order first.  The
 * value is a positive offset from the opcode of the node containing it.
 * An operand, if any, simply follows the node.  (Note that much of the
 * code generation knows about this implicit relationship.)
 *
 * Using two bytes for the "next" pointer is vast overkill for most things,
 * but allows patterns to get big without disasters.
 */
#define        OP(p)   (*(p))
#define        NEXT(p) (((*((p)+1)&0377)<<8) + (*((p)+2)&0377))
#define        OPERAND(p)      ((p) + 3)

/*
 * See regmagic.h for one further detail of program structure.
 */


/*
 * Utility definitions.
 */
#ifndef CHARBITS
#define        UCHARAT(p)      ((int)*(unsigned char *)(p))
#else
#define        UCHARAT(p)      ((int)*(p)&CHARBITS)
#endif

#define        FAIL(m) { regerror(m); return(NULL); }
#define        ISMULT(c)       ((c) == '*' || (c) == '+' || (c) == '?')
#define        META    "^$.[()|?+*\\"

/*
 * Flags to be passed up and down.
 */
#define        HASWIDTH        01      /* Known never to match null string. */
#define        SIMPLE          02      /* Simple enough to be STAR/PLUS operand. */
#define        SPSTART         04      /* Starts with * or +. */
#define        WORST           0       /* Worst case. */

/*
 * Global work variables for regcomp().
 */
static char *regparse;         /* Input-scan pointer. */
static int regnpar;            /* () count. */
static char regdummy;
static char *regcode;          /* Code-emit pointer; &regdummy = don't. */
static long regsize;           /* Code size. */
#ifdef MULTILINE
static int regnl;              /* '\n' detected. */
#endif

/*
 * Forward declarations for regcomp()'s friends.
 */
#ifndef STATIC
#define        STATIC  static
#endif
STATIC char *reg();
STATIC char *regbranch();
STATIC char *regpiece();
STATIC char *regatom();
STATIC char *regnode();
STATIC char *regnext();
STATIC void regc();
STATIC void reginsert();
STATIC void regtail();
STATIC void regoptail();
#ifdef STRCSPN
STATIC int strcspn();
#endif

/*
 - regcomp - compile a regular expression into internal code
 *
 * We can't allocate space until we know how big the compiled form will be,
 * but we can't compile it (and thus know how big it is) until we've got a
 * place to put the code.  So we cheat:  we compile it twice, once with code
 * generation turned off and size counting turned on, and once "for real".
 * This also means that we don't allocate space until we are sure that the
 * thing really will compile successfully, and we never have to move the
 * code and thus invalidate pointers into it.  (Note that it has to be in
 * one piece because free() must be able to free it all.)
 *
 * Beware that the optimization-preparation code in here knows about some
 * of the structure of the compiled regexp.
 */
regexp *
regcomp(exp)
char *exp;
{
       register regexp *r;
       register char *scan;
       register char *longest;
       register int len;
       int flags;

       if (exp == NULL)
               FAIL("NULL argument");

       /* First pass: determine size, legality. */
       regparse = exp;
       regnpar = 1;
       regsize = 0L;
       regcode = &regdummy;
#ifdef MULTILINE
       regnl = 0;
#endif
       regc(MAGIC);
       if (reg(0, &flags) == NULL)
               return(NULL);

       /* Small enough for pointer-storage convention? */
       if (regsize >= 32767L)               /* Probably could be 65535L. */
               FAIL("regexp too big");

       /* Allocate space. */
       r = (regexp *)malloc(sizeof(regexp) + (unsigned)regsize);
       if (r == NULL)
               FAIL("out of space");

       /* Second pass: emit code. */
       regparse = exp;
       regnpar = 1;
       regcode = r->program;
       regc(MAGIC);
       if (reg(0, &flags) == NULL)
               return(NULL);

       /* Dig out information for optimizations. */
       r->regstart = '\0';  /* Worst-case defaults. */
       r->reganch = 0;
       r->regmust = NULL;
       r->regmlen = 0;
       scan = r->program+1;                 /* First BRANCH. */
       if (OP(regnext(scan)) == END) {         /* Only one top-level choice. */
               scan = OPERAND(scan);

               /* Starting-point info. */
               if (OP(scan) == EXACTLY)
                       r->regstart = *OPERAND(scan);
               else if (OP(scan) == BOL)
                       r->reganch++;

               /*
                * If there's something expensive in the r.e., find the
                * longest literal string that must appear and make it the
                * regmust.  Resolve ties in favor of later strings, since
                * the regstart check works with the beginning of the r.e.
                * and avoiding duplication strengthens checking.  Not a
                * strong reason, but sufficient in the absence of others.
                */
#ifdef MULTILINE
               if ((flags&SPSTART) && !regnl) {
#else
               if (flags&SPSTART) {
#endif
                       longest = NULL;
                       len = 0;
                       for (; scan != NULL; scan = regnext(scan))
                               if (OP(scan) == EXACTLY && strlen(OPERAND(scan)) >= len) {
                                       longest = OPERAND(scan);
                                       len = strlen(OPERAND(scan));
                               }
                       r->regmust = longest;
                       r->regmlen = len;
               }
       }

       return(r);
}

/*
 - reg - regular expression, i.e. main body or parenthesized thing
 *
 * Caller must absorb opening parenthesis.
 *
 * Combining parenthesis handling with the base level of regular expression
 * is a trifle forced, but the need to tie the tails of the branches to what
 * follows makes it hard to avoid.
 */
static char *
reg(paren, flagp)
int paren;                     /* Parenthesized? */
int *flagp;
{
       register char *ret;
       register char *br;
       register char *ender;
       register int parno;
       int flags;

       *flagp = HASWIDTH;      /* Tentatively. */

       /* Make an OPEN node, if parenthesized. */
       if (paren) {
               if (regnpar >= NSUBEXP)
                       FAIL("too many ()");
               parno = regnpar;
               regnpar++;
               ret = regnode(OPEN+parno);
       } else
               ret = NULL;

       /* Pick up the branches, linking them together. */
       br = regbranch(&flags);
       if (br == NULL)
               return(NULL);
       if (ret != NULL)
               regtail(ret, br);       /* OPEN -> first. */
       else
               ret = br;
       if (!(flags&HASWIDTH))
               *flagp &= ~HASWIDTH;
       *flagp |= flags&SPSTART;
       while (*regparse == '|') {
               regparse++;
               br = regbranch(&flags);
               if (br == NULL)
                       return(NULL);
               regtail(ret, br);       /* BRANCH -> BRANCH. */
               if (!(flags&HASWIDTH))
                       *flagp &= ~HASWIDTH;
               *flagp |= flags&SPSTART;
       }

       /* Make a closing node, and hook it on the end. */
       ender = regnode((paren) ? CLOSE+parno : END);
       regtail(ret, ender);

       /* Hook the tails of the branches to the closing node. */
       for (br = ret; br != NULL; br = regnext(br))
               regoptail(br, ender);

       /* Check for proper termination. */
       if (paren && *regparse++ != ')') {
               FAIL("unmatched ()");
       } else if (!paren && *regparse != '\0') {
               if (*regparse == ')') {
                       FAIL("unmatched ()");
               } else
                       FAIL("junk on end");  /* "Can't happen". */
               /* NOTREACHED */
       }

       return(ret);
}

/*
 - regbranch - one alternative of an | operator
 *
 * Implements the concatenation operator.
 */
static char *
regbranch(flagp)
int *flagp;
{
       register char *ret;
       register char *chain;
       register char *latest;
       int flags;

       *flagp = WORST;         /* Tentatively. */

       ret = regnode(BRANCH);
       chain = NULL;
       while (*regparse != '\0' && *regparse != '|' && *regparse != ')') {
               latest = regpiece(&flags);
               if (latest == NULL)
                       return(NULL);
               *flagp |= flags&HASWIDTH;
               if (chain == NULL)      /* First piece. */
                       *flagp |= flags&SPSTART;
               else
                       regtail(chain, latest);
               chain = latest;
       }
       if (chain == NULL)      /* Loop ran zero times. */
               (void) regnode(NOTHING);

       return(ret);
}

/*
 - regpiece - something followed by possible [*+?]
 *
 * Note that the branching code sequences used for ? and the general cases
 * of * and + are somewhat optimized:  they use the same NOTHING node as
 * both the endmarker for their branch list and the body of the last branch.
 * It might seem that this node could be dispensed with entirely, but the
 * endmarker role is not redundant.
 */
static char *
regpiece(flagp)
int *flagp;
{
       register char *ret;
       register char op;
       register char *next;
       int flags;

       ret = regatom(&flags);
       if (ret == NULL)
               return(NULL);

       op = *regparse;
       if (!ISMULT(op)) {
               *flagp = flags;
               return(ret);
       }

       if (!(flags&HASWIDTH) && op != '?')
               FAIL("*+ operand could be empty");
       *flagp = (op != '+') ? (WORST|SPSTART) : (WORST|HASWIDTH);

       if (op == '*' && (flags&SIMPLE))
               reginsert(STAR, ret);
       else if (op == '*') {
               /* Emit x* as (x&|), where & means "self". */
               reginsert(BRANCH, ret);                 /* Either x */
               regoptail(ret, regnode(BACK));          /* and loop */
               regoptail(ret, ret);                    /* back */
               regtail(ret, regnode(BRANCH));          /* or */
               regtail(ret, regnode(NOTHING));         /* null. */
       } else if (op == '+' && (flags&SIMPLE))
               reginsert(PLUS, ret);
       else if (op == '+') {
               /* Emit x+ as x(&|), where & means "self". */
               next = regnode(BRANCH);                 /* Either */
               regtail(ret, next);
               regtail(regnode(BACK), ret);            /* loop back */
               regtail(next, regnode(BRANCH));         /* or */
               regtail(ret, regnode(NOTHING));         /* null. */
       } else if (op == '?') {
               /* Emit x? as (x|) */
               reginsert(BRANCH, ret);                 /* Either x */
               regtail(ret, regnode(BRANCH));          /* or */
               next = regnode(NOTHING);                /* null. */
               regtail(ret, next);
               regoptail(ret, next);
       }
       regparse++;
       if (ISMULT(*regparse))
               FAIL("nested *?+");

       return(ret);
}

/*
 - regatom - the lowest level
 *
 * Optimization:  gobbles an entire sequence of ordinary characters so that
 * it can turn them into a single node, which is smaller to store and
 * faster to run.  Backslashed characters are exceptions, each becoming a
 * separate node; the code is simpler that way and it's not worth fixing.
 */
static char *
regatom(flagp)
int *flagp;
{
       register char *ret;
       int flags;

       *flagp = WORST;         /* Tentatively. */

       switch (*regparse++) {
       case '^':
               ret = regnode(BOL);
               break;
       case '$':
               ret = regnode(EOL);
               break;
       case '.':
               ret = regnode(ANY);
               *flagp |= HASWIDTH|SIMPLE;
               break;
       case '[': {
                       register int class;
                       register int classend;

                       if (*regparse == '^') { /* Complement of range. */
                               ret = regnode(ANYBUT);
                               regparse++;
                       } else
                               ret = regnode(ANYOF);
                       if (*regparse == ']' || *regparse == '-')
                               regc(*regparse++);
                       while (*regparse != '\0' && *regparse != ']') {
                               if (*regparse == '-') {
                                       regparse++;
                                       if (*regparse == ']' || *regparse == '\0')
                                               regc('-');
                                       else {
                                               class = UCHARAT(regparse-2)+1;
                                               classend = UCHARAT(regparse);
                                               if (class > classend+1)
                                                       FAIL("invalid [] range");
                                               for (; class <= classend; class++)
                                                       regc(class);
                                               regparse++;
                                       }
                               } else
                                       regc(*regparse++);
                       }
                       regc('\0');
                       if (*regparse != ']')
                               FAIL("unmatched []");
                       regparse++;
                       *flagp |= HASWIDTH|SIMPLE;
               }
               break;
       case '(':
               ret = reg(1, &flags);
               if (ret == NULL)
                       return(NULL);
               *flagp |= flags&(HASWIDTH|SPSTART);
               break;
       case '\0':
       case '|':
       case ')':
               FAIL("internal urp"); /* Supposed to be caught earlier. */
               break;
       case '?':
       case '+':
       case '*':
               FAIL("?+* follows nothing");
               break;
       case '\\':
               if (*regparse == '\0')
                       FAIL("trailing \\");
               ret = regnode(EXACTLY);
#ifdef MULTILINE
               if (*regparse == 'n') {
                       regc('\n');
                       regparse++;
                       regnl++;
               }
               else
#endif
               regc(*regparse++);
               regc('\0');
               *flagp |= HASWIDTH|SIMPLE;
               break;
       default: {
                       register int len;
                       register char ender;

                       regparse--;
                       len = strcspn(regparse, META);
                       if (len <= 0)
                               FAIL("internal disaster");
                       ender = *(regparse+len);
                       if (len > 1 && ISMULT(ender))
                               len--;          /* Back off clear of ?+* operand. */
                       *flagp |= HASWIDTH;
                       if (len == 1)
                               *flagp |= SIMPLE;
                       ret = regnode(EXACTLY);
                       while (len > 0) {
#ifdef MULTILINE
                               if (*regparse == '\n')
                                       regnl++;
#endif
                               regc(*regparse++);
                               len--;
                       }
                       regc('\0');
               }
               break;
       }

       return(ret);
}

/*
 - regnode - emit a node
 */
static char *                  /* Location. */
regnode(op)
char op;
{
       register char *ret;
       register char *ptr;

       ret = regcode;
       if (ret == &regdummy) {
               regsize += 3;
               return(ret);
       }

       ptr = ret;
       *ptr++ = op;
       *ptr++ = '\0';          /* Null "next" pointer. */
       *ptr++ = '\0';
       regcode = ptr;

       return(ret);
}

/*
 - regc - emit (if appropriate) a byte of code
 */
static void
regc(b)
char b;
{
       if (regcode != &regdummy)
               *regcode++ = b;
       else
               regsize++;
}

/*
 - reginsert - insert an operator in front of already-emitted operand
 *
 * Means relocating the operand.
 */
static void
reginsert(op, opnd)
char op;
char *opnd;
{
       register char *src;
       register char *dst;
       register char *place;

       if (regcode == &regdummy) {
               regsize += 3;
               return;
       }

       src = regcode;
       regcode += 3;
       dst = regcode;
       while (src > opnd)
               *--dst = *--src;

       place = opnd;           /* Op node, where operand used to be. */
       *place++ = op;
       *place++ = '\0';
       *place++ = '\0';
}

/*
 - regtail - set the next-pointer at the end of a node chain
 */
static void
regtail(p, val)
char *p;
char *val;
{
       register char *scan;
       register char *temp;
       register int offset;

       if (p == &regdummy)
               return;

       /* Find last node. */
       scan = p;
       for (;;) {
               temp = regnext(scan);
               if (temp == NULL)
                       break;
               scan = temp;
       }

       if (OP(scan) == BACK)
               offset = scan - val;
       else
               offset = val - scan;
       *(scan+1) = (offset>>8)&0377;
       *(scan+2) = offset&0377;
}

/*
 - regoptail - regtail on operand of first argument; nop if operandless
 */
static void
regoptail(p, val)
char *p;
char *val;
{
       /* "Operandless" and "op != BRANCH" are synonymous in practice. */
       if (p == NULL || p == &regdummy || OP(p) != BRANCH)
               return;
       regtail(OPERAND(p), val);
}

/*
 * regexec and friends
 */

/*
 * Global work variables for regexec().
 */
static char *reginput;         /* String-input pointer. */
static char *regbol;           /* Beginning of input, for ^ check. */
static char **regstartp;       /* Pointer to startp array. */
static char **regendp;         /* Ditto for endp. */

/*
 * Forwards.
 */
STATIC int regtry();
STATIC int regmatch();
STATIC int regrepeat();

#ifdef DEBUG
int regnarrate = 0;
void regdump();
STATIC char *regprop();
#endif

/*
 - regexec - match a regexp against a string
 */
int
regexec(prog, string)
register regexp *prog;
register char *string;
{
       register char *s;
       extern char *strchr();

       /* Be paranoid... */
       if (prog == NULL || string == NULL) {
               regerror("NULL parameter");
               return(0);
       }

#ifdef MULTILINE
       /* Check for \n in string, and if so, call the more general routine. */
       if (strchr(string, '\n') != NULL)
               return reglexec(prog, string, 0);
#endif

       /* Check validity of program. */
       if (UCHARAT(prog->program) != MAGIC) {
               regerror("corrupted program");
               return(0);
       }

       /* If there is a "must appear" string, look for it. */
       if (prog->regmust != NULL) {
               s = string;
               while ((s = strchr(s, prog->regmust[0])) != NULL) {
                       if (strncmp(s, prog->regmust, prog->regmlen) == 0)
                               break;  /* Found it. */
                       s++;
               }
               if (s == NULL)  /* Not present. */
                       return(0);
       }

       /* Mark beginning of line for ^ . */
       regbol = string;

       /* Simplest case:  anchored match need be tried only once. */
       if (prog->reganch)
               return(regtry(prog, string));

       /* Messy cases:  unanchored match. */
       s = string;
       if (prog->regstart != '\0')
               /* We know what char it must start with. */
               while ((s = strchr(s, prog->regstart)) != NULL) {
                       if (regtry(prog, s))
                               return(1);
                       s++;
               }
       else
               /* We don't -- general case. */
               do {
                       if (regtry(prog, s))
                               return(1);
               } while (*s++ != '\0');

       /* Failure. */
       return(0);
}

#ifdef MULTILINE
/*
 - reglexec - match a regexp against a long string buffer, starting at offset
 */
int
reglexec(prog, string, offset)
register regexp *prog;
register char *string;
{
       register char *s;
       extern char *strchr();

       /* Be paranoid... */
       if (prog == NULL || string == NULL) {
               regerror("NULL parameter");
               return(0);
       }

       /* Check validity of program. */
       if (UCHARAT(prog->program) != MAGIC) {
               regerror("corrupted program");
               return(0);
       }

       /* (Don't look for "must appear" string -- string can be long.) */

       /* Mark beginning of line for ^ . */
       regbol = string;

       /* Apply offset.
          Assume 0 <= offset <= strlen(string), but don't check,
          as string can be long. */
       s= string + offset;

       /* Anchored match need be tried only at line starts. */
       if (prog->reganch) {
               while (!regtry(prog, s)) {
                       s = strchr(s, '\n');
                       if (s == NULL)
                               return(0);
                       s++;
               }
               return(1);
       }

       /* Messy cases:  unanchored match. */
       if (prog->regstart != '\0')
               /* We know what char it must start with. */
               while ((s = strchr(s, prog->regstart)) != NULL) {
                       if (regtry(prog, s))
                               return(1);
                       s++;
               }
       else
               /* We don't -- general case. */
               do {
                       if (regtry(prog, s))
                               return(1);
               } while (*s++ != '\0');

       /* Failure. */
       return(0);
}
#endif

/*
 - regtry - try match at specific point
 */
static int                     /* 0 failure, 1 success */
regtry(prog, string)
regexp *prog;
char *string;
{
       register int i;
       register char **sp;
       register char **ep;

       reginput = string;
       regstartp = prog->startp;
       regendp = prog->endp;

       sp = prog->startp;
       ep = prog->endp;
       for (i = NSUBEXP; i > 0; i--) {
               *sp++ = NULL;
               *ep++ = NULL;
       }
       if (regmatch(prog->program + 1)) {
               prog->startp[0] = string;
               prog->endp[0] = reginput;
               return(1);
       } else
               return(0);
}

/*
 - regmatch - main matching routine
 *
 * Conceptually the strategy is simple:  check to see whether the current
 * node matches, call self recursively to see whether the rest matches,
 * and then act accordingly.  In practice we make some effort to avoid
 * recursion, in particular by going through "ordinary" nodes (that don't
 * need to know whether the rest of the match failed) by a loop instead of
 * by recursion.
 */
static int                     /* 0 failure, 1 success */
regmatch(prog)
char *prog;
{
       register char *scan;    /* Current node. */
       char *next;             /* Next node. */
       extern char *strchr();

       scan = prog;
#ifdef DEBUG
       if (scan != NULL && regnarrate)
               fprintf(stderr, "%s(\n", regprop(scan));
#endif
       while (scan != NULL) {
#ifdef DEBUG
               if (regnarrate)
                       fprintf(stderr, "%s...\n", regprop(scan));
#endif
               next = regnext(scan);

               switch (OP(scan)) {
               case BOL:
#ifdef MULTILINE
                       if (!(reginput == regbol ||
                               reginput > regbol && *(reginput-1) == '\n'))
#else
                       if (reginput != regbol)
#endif
                               return(0);
                       break;
               case EOL:
#ifdef MULTILINE
                       if (*reginput != '\0' && *reginput != '\n')
#else
                       if (*reginput != '\0')
#endif
                               return(0);
                       break;
               case ANY:
#ifdef MULTILINE
                       if (*reginput == '\0' || *reginput == '\n')
#else
                       if (*reginput == '\0')
#endif
                               return(0);
                       reginput++;
                       break;
               case EXACTLY: {
                               register int len;
                               register char *opnd;

                               opnd = OPERAND(scan);
                               /* Inline the first character, for speed. */
                               if (*opnd != *reginput)
                                       return(0);
                               len = strlen(opnd);
                               if (len > 1 && strncmp(opnd, reginput, len) != 0)
                                       return(0);
                               reginput += len;
                       }
                       break;
               case ANYOF:
                       if (*reginput == '\0' || strchr(OPERAND(scan), *reginput) == NULL)
                               return(0);
                       reginput++;
                       break;
               case ANYBUT:
#ifdef MULTILINE
                       if (*reginput == '\0' || *reginput == '\n' ||
                               strchr(OPERAND(scan), *reginput) != NULL)
#else
                       if (*reginput == '\0' || strchr(OPERAND(scan), *reginput) != NULL)
#endif
                               return(0);
                       reginput++;
                       break;
               case NOTHING:
                       break;
               case BACK:
                       break;
               case OPEN+1:
               case OPEN+2:
               case OPEN+3:
               case OPEN+4:
               case OPEN+5:
               case OPEN+6:
               case OPEN+7:
               case OPEN+8:
               case OPEN+9: {
                               register int no;
                               register char *save;

                               no = OP(scan) - OPEN;
                               save = reginput;

                               if (regmatch(next)) {
                                       /*
                                        * Don't set startp if some later
                                        * invocation of the same parentheses
                                        * already has.
                                        */
                                       if (regstartp[no] == NULL)
                                               regstartp[no] = save;
                                       return(1);
                               } else
                                       return(0);
                       }
                       break;
               case CLOSE+1:
               case CLOSE+2:
               case CLOSE+3:
               case CLOSE+4:
               case CLOSE+5:
               case CLOSE+6:
               case CLOSE+7:
               case CLOSE+8:
               case CLOSE+9: {
                               register int no;
                               register char *save;

                               no = OP(scan) - CLOSE;
                               save = reginput;

                               if (regmatch(next)) {
                                       /*
                                        * Don't set endp if some later
                                        * invocation of the same parentheses
                                        * already has.
                                        */
                                       if (regendp[no] == NULL)
                                               regendp[no] = save;
                                       return(1);
                               } else
                                       return(0);
                       }
                       break;
               case BRANCH: {
                               register char *save;

                               if (OP(next) != BRANCH)         /* No choice. */
                                       next = OPERAND(scan);   /* Avoid recursion. */
                               else {
                                       do {
                                               save = reginput;
                                               if (regmatch(OPERAND(scan)))
                                                       return(1);
                                               reginput = save;
                                               scan = regnext(scan);
                                       } while (scan != NULL && OP(scan) == BRANCH);
                                       return(0);
                                       /* NOTREACHED */
                               }
                       }
                       break;
               case STAR:
               case PLUS: {
                               register char nextch;
                               register int no;
                               register char *save;
                               register int min;

                               /*
                                * Lookahead to avoid useless match attempts
                                * when we know what character comes next.
                                */
                               nextch = '\0';
                               if (OP(next) == EXACTLY)
                                       nextch = *OPERAND(next);
                               min = (OP(scan) == STAR) ? 0 : 1;
                               save = reginput;
                               no = regrepeat(OPERAND(scan));
                               while (no >= min) {
                                       /* If it could work, try it. */
                                       if (nextch == '\0' || *reginput == nextch)
                                               if (regmatch(next))
                                                       return(1);
                                       /* Couldn't or didn't -- back up. */
                                       no--;
                                       reginput = save + no;
                               }
                               return(0);
                       }
                       break;
               case END:
                       return(1);      /* Success! */
                       break;
               default:
                       regerror("memory corruption");
                       return(0);
                       break;
               }

               scan = next;
       }

       /*
        * We get here only if there's trouble -- normally "case END" is
        * the terminating point.
        */
       regerror("corrupted pointers");
       return(0);
}

/*
 - regrepeat - repeatedly match something simple, report how many
 */
static int
regrepeat(p)
char *p;
{
       register int count = 0;
       register char *scan;
       register char *opnd;
#ifdef MULTILINE
       register char *eol;
#endif

       scan = reginput;
       opnd = OPERAND(p);
       switch (OP(p)) {
       case ANY:
#ifdef MULTILINE
               if ((eol = strchr(scan, '\n')) != NULL) {
                       count += eol - scan;
                       scan = eol;
                       break;
               }
#endif
               count = strlen(scan);
               scan += count;
               break;
       case EXACTLY:
               while (*opnd == *scan) {
                       count++;
                       scan++;
               }
               break;
       case ANYOF:
               while (*scan != '\0' && strchr(opnd, *scan) != NULL) {
                       count++;
                       scan++;
               }
               break;
       case ANYBUT:
#ifdef MULTILINE
               while (*scan != '\0' && *scan != '\n' &&
                       strchr(opnd, *scan) == NULL) {
#else
               while (*scan != '\0' && strchr(opnd, *scan) == NULL) {
#endif
                       count++;
                       scan++;
               }
               break;
       default:                /* Oh dear.  Called inappropriately. */
               regerror("internal foulup");
               count = 0;      /* Best compromise. */
               break;
       }
       reginput = scan;

       return(count);
}

/*
 - regnext - dig the "next" pointer out of a node
 */
static char *
regnext(p)
register char *p;
{
       register int offset;

       if (p == &regdummy)
               return(NULL);

       offset = NEXT(p);
       if (offset == 0)
               return(NULL);

       if (OP(p) == BACK)
               return(p-offset);
       else
               return(p+offset);
}

#ifdef DEBUG

STATIC char *regprop();

/*
 - regdump - dump a regexp onto stdout in vaguely comprehensible form
 */
void
regdump(r)
regexp *r;
{
       register char *s;
       register char op = EXACTLY;     /* Arbitrary non-END op. */
       register char *next;
       extern char *strchr();


       s = r->program + 1;
       while (op != END) {     /* While that wasn't END last time... */
               op = OP(s);
               printf("%2d%s", (int)(s-r->program), regprop(s));  /* Where, what. */
               next = regnext(s);
               if (next == NULL)               /* Next ptr. */
                       printf("(0)");
               else
                       printf("(%d)", (int)((s-r->program)+(next-s)));
               s += 3;
               if (op == ANYOF || op == ANYBUT || op == EXACTLY) {
                       /* Literal string, where present. */
                       while (*s != '\0') {
#ifdef MULTILINE
                               if (*s == '\n')
                                       printf("\\n");
                               else
#endif
                               putchar(*s);
                               s++;
                       }
                       s++;
               }
               putchar('\n');
       }

       /* Header fields of interest. */
       if (r->regstart != '\0')
               printf("start `%c' ", r->regstart);
       if (r->reganch)
               printf("anchored ");
       if (r->regmust != NULL)
               printf("must have \"%s\"", r->regmust);
       printf("\n");
}

/*
 - regprop - printable representation of opcode
 */
static char *
regprop(op)
char *op;
{
       register char *p;
       static char buf[50];

       (void) strcpy(buf, ":");

       switch (OP(op)) {
       case BOL:
               p = "BOL";
               break;
       case EOL:
               p = "EOL";
               break;
       case ANY:
               p = "ANY";
               break;
       case ANYOF:
               p = "ANYOF";
               break;
       case ANYBUT:
               p = "ANYBUT";
               break;
       case BRANCH:
               p = "BRANCH";
               break;
       case EXACTLY:
               p = "EXACTLY";
               break;
       case NOTHING:
               p = "NOTHING";
               break;
       case BACK:
               p = "BACK";
               break;
       case END:
               p = "END";
               break;
       case OPEN+1:
       case OPEN+2:
       case OPEN+3:
       case OPEN+4:
       case OPEN+5:
       case OPEN+6:
       case OPEN+7:
       case OPEN+8:
       case OPEN+9:
               sprintf(buf+strlen(buf), "OPEN%d", (int)(OP(op)-OPEN));
               p = NULL;
               break;
       case CLOSE+1:
       case CLOSE+2:
       case CLOSE+3:
       case CLOSE+4:
       case CLOSE+5:
       case CLOSE+6:
       case CLOSE+7:
       case CLOSE+8:
       case CLOSE+9:
               sprintf(buf+strlen(buf), "CLOSE%d", (int)(OP(op)-CLOSE));
               p = NULL;
               break;
       case STAR:
               p = "STAR";
               break;
       case PLUS:
               p = "PLUS";
               break;
       default:
               regerror("corrupted opcode");
               break;
       }
       if (p != NULL)
               (void) strcat(buf, p);
       return(buf);
}
#endif

/*
 * The following is provided for those people who do not have strcspn() in
 * their C libraries.  They should get off their butts and do something
 * about it; at least one public-domain implementation of those (highly
 * useful) string routines has been published on Usenet.
 */
#ifdef STRCSPN
/*
 * strcspn - find length of initial segment of s1 consisting entirely
 * of characters not from s2
 */

static int
strcspn(s1, s2)
char *s1;
char *s2;
{
       register char *scan1;
       register char *scan2;
       register int count;

       count = 0;
       for (scan1 = s1; *scan1 != '\0'; scan1++) {
               for (scan2 = s2; *scan2 != '\0';)       /* ++ moved down. */
                       if (*scan1 == *scan2++)
                               return(count);
               count++;
       }
       return(count);
}
#endif
